Rotary drag bits employing superabrasive cutting elements in the form of polycrystalline diamond compact (PDC) cutters have been employed for several decades. PDC cutters are typically comprised of a disc-shaped diamond “table” formed on and bonded under high-pressure and high-temperature conditions to a supporting substrate such as cemented tungsten carbide (WC), although other configurations are known. Bits carrying PDC cutters, which for example, may be brazed into pockets in the bit face, pockets in blades extending from the face, or mounted to studs inserted into the bit body, have proven very effective in achieving high rates of penetration (ROP) in drilling subterranean formations exhibiting low to medium compressive strengths. Recent improvements in the design of hydraulic flow regimes about the face of bits, cutter design, and drilling fluid formulation have reduced prior, notable tendencies of such bits to “ball” by increasing the volume of formation material which may be cut before exceeding the ability of the bit and its associated drilling fluid flow to clear the formation cuttings from the bit face.
Even in view of such improvements, however, PDC cutters still suffer from what might simply be termed “overloading” even at low weight-on-bit (WOB) applied to the drill string to which the bit carrying such cutters is mounted, especially if aggressive cutting structures are employed. The relationship of torque to WOB may be employed as an indicator of aggressivity for cutters, so the higher the torque to WOB ratio, the more aggressive the bit. The problem of excessive bit aggressiveness is particularly significant in low compressive strength formations where an unduly great depth of cut (DOC) may be achieved at extremely low WOB. The problem may also be aggravated by drill string bounce, wherein the elasticity of the drill string may cause erratic application of WOB to the drill bit, with consequent overloading. Moreover, operating PDC cutters at an excessively high DOC may generate more formation cuttings than can be consistently cleared from the bit face and back up the bore hole via the junk slots on the face of the bit by even the aforementioned improved, state-of-the-art bit hydraulics, leading to the aforementioned bit balling phenomenon.
Another, separate problem involves drilling from a zone or stratum of higher formation compressive strength to a “softer” zone of lower compressive strength. As the bit drills into the softer formation without changing the applied WOB (or before the WOB can be reduced by the driller), the penetration of the PDC cutters, and thus the resulting torque on the bit (TOB), increase almost instantaneously and by a substantial magnitude. The abruptly higher torque, in turn, may cause damage to the cutters and/or the bit body itself. In directional drilling, such a change causes the tool face orientation of the directional (measuring-while-drilling, or MWD, or a steering tool) assembly to fluctuate, making it more difficult for the directional driller to follow the planned directional path for the bit. Thus, it may be necessary for the directional driller to back off the bit from the bottom of the borehole to reset or reorient the tool face. In addition, a downhole motor, such as drilling fluid-driven Moineau-type motors commonly employed in directional drilling operations in combination with a steerable bottomhole assembly, may completely stall under a sudden torque increase. That is, the bit may stop rotating, thereby stopping the drilling operation and again necessitating backing off the bit from the borehole bottom to re-establish drilling fluid flow and motor output. Such interruptions in the drilling of a well can be time consuming and quite costly.
Numerous attempts using varying approaches have been made over the years to protect the integrity of diamond cutters and their mounting structures and to limit cutter penetration into a formation being drilled. For example, from a period even before the advent of commercial use of PDC cutters, U.S. Pat. No. 3,709,308 discloses the use of trailing, round natural diamonds on the bit body to limit the penetration of cubic diamonds employed to cut a formation. U.S. Pat. No. 4,351,401 discloses the use of surface set natural diamonds at or near the gage of the bit as penetration limiters to control the depth-of-cut of PDC cutters on the bit face. The following other patents disclose the use of a variety of structures immediately trailing PDC cutters (with respect to the intended direction of bit rotation) to protect the cutters or their mounting structures: U.S. Pat. Nos. 4,889,017; 4,991,670; 5,244,039 and 5,303,785. U.S. Pat. No. 5,314,033 discloses, inter alia, the use of cooperating positive and negative or neutral backrake cutters to limit penetration of the positive rake cutters into the formation. Another approach to limiting cutting element penetration is to employ structures or features on the bit body rotationally preceding (rather than trailing) PDC cutters, as disclosed in U.S. Pat. Nos. 3,153,458; 4,554,986; 5,199,511 and 5,595,252.
In another context, that of so-called “anti-whirl” drilling structures, it has been asserted in U.S. Pat. No. 5,402,856 that a bearing surface aligned with a resultant radial force generated by an anti-whirl underreamer should be sized so that force per area applied to the borehole sidewall will not exceed the compressive strength of the formation being underreamed. See also U.S. Pat. Nos. 4,982,802; 5,010,789; 5,042,596; 5,111,892 and 5,131,478.
While some of the foregoing patents recognize the desirability to limit cutter penetration, or DOC, or otherwise limit forces applied to a borehole surface, the disclosed approaches are somewhat generalized in nature and fail to accommodate or implement an engineered approach to achieving a target ROP in combination with more stable, predictable bit performance. Furthermore, the disclosed approaches do not provide a bit or method of drilling which is generally tolerant to being axially loaded with an amount of weight-on-bit over and in excess what would be optimum for the current rate-of-penetration for the particular formation being drilled and which would not generate high amounts of potentially bit-stopping or bit-damaging torque-on-bit should the bit nonetheless be subjected to such excessive amounts of weight-on-bit.
Various successful solutions to the problem of excessive cutter penetration are presented in U.S. Pat. Nos. 6,298,930; 6,460,631; 6,779,613 and 6,935,441, the disclosure of each of which is incorporated by reference in its entirety herein. Specifically, U.S. Pat. No. 6,298,930 describes a rotary drag bit including exterior features to control the depth of cut by cutters mounted thereon, so as to control the volume of formation material cut per bit rotation as well as the torque experienced by the bit and an associated bottom-hole assembly. These features, also termed depth of cut control (DOCC) features, provide the bearing surface or sufficient surface area to withstand the axial or longitudinal WOB without exceeding the compressive strength of the formation being drilled and such that the depth of penetration of PDC cutters cutting into the formation is controlled. Because the DOCC features are subject to the applied WOB as well as to contact with the abrasive formation and abrasives-laden drilling fluids, the DOCC features may be layered onto the surface of a steel body bit as an appliqué or hard face weld having the material characteristics required for a high load and high abrasion/erosion environment, or include individual, discrete wear resistant elements or inserts set in bearing surfaces cast in the face of a matrix-type bit, as depicted in FIG. 1 of U.S. Pat. No 6,298,930. The wear resistant inserts or elements may comprise tungsten carbide bricks or discs, diamond grit, diamond film, natural or synthetic diamond (PDC or TSP), or cubic boron nitride.
FIGS. 10A and 10B of the '930 patent, respectively, depict different DOCC feature and PDC cutter combinations. In each instance, a single PDC cutter is secured to a combined cutter carrier and DOC limiter, the carrier then being received within a cavity in the face (or on a blade) of a bit and secured therein. The DOC limiter includes a protrusion exhibiting a bearing surface.
While the DOCC features are extremely advantageous for limiting a depth of cut while managing a given WOB, the manufacture of the depth of cut control features upon the bit requires: 1) labor intensive manufacturing to necessarily obtain the precise or desired amount of layered hard facing required for a particular or designed target depth of cut (TDOC) or 2) complicated manufacturing processes to form the bit body in order to assemble and secure each combined cutter carrier having a single PDC cutter and associated DOC limiter placed into a cavity in the face or on a blade of the bit body. Moreover, the foregoing patents do not provide a bit wherein the TDOC and the designed bearing (which may also be termed “rubbing”) surface area, i.e., potential contact area with the “to be” drilled subterranean formation, are simultaneously provided for in a structure selectively attachable to a given bit frame, in order to provide variety and selectability of the TDOC and the designed rubbing surface area with a high degree of precision for the given bit frame.
Moreover, many steel body PDC bits are manufactured by cutting the whole blade profile and, in some instances, an entire bit body including the blades, from a material, such as a steel or other casting, with cutter pockets milled into the blades, which are assembled to obtain the bit body or frame, which is then selectively manually hardfaced to create an abrasion-resistant layer for a bearing or rubbing surface. The hardfacing invariably has a tolerance that is either below the amount required for reduced exposure or beyond the amount required for DOCC features. Also, the hardfacing does not provide a precise or controlled rubbing surface area. Further, the hardfacing is permanent as applied and requires grinding in order to remove or modify its thickness when applied beyond an acceptable tolerance.
While matrix body bits are formed by machining features into a mold and provide other features using so-called displacements which are inserted into the mold cavity, achieving precise exposure for cutters within the cone of such a bit body may be difficult due to the angular orientation of the required machining, as well as variances attributable to warpage and shrinkage of the bit body during cooling after infiltration with a molten metal alloy binder. Relatively larger bit bodies may exhibit more variance from the intended dimensions.
Accordingly, it is desirable to provide a bit that eliminates the manufacturing uncertainty or complexity required in obtaining a given TDOC. Also, it is desirable to provide a bit that allows for a selectable bearing or rubbing surface area without, or not requiring, alteration to the bit frame. Moreover, it is desirable to provide TDOC and/or rubbing surface area selectabilty for a given bit frame, providing for inventory reduction of bit frames and allowing for less complicated refabrication or repair of the drill bit to achieve a different TDOC and/or rubbing surface area. Further, it is desirable on steel body bits to achieve an extremely accurate TDOC and/or rubbing surface area while allowing manufacture of bits, i.e., their bit frames, with more accuracy than otherwise provided by hardfacing, in order to provide increased precision of cutter exposure and controlled rubbing area thereof. Furthermore, in providing for the selectability of the rubbing surface area and thickness, it is desirable to provide designed abrasion resistance to enhance the bit's life by limiting, i.e., controlling, wear caused by rubbing surface contact during drilling. Finally, it is desirable to provide the above desired improvements affording increased reparability, inventory flexibility (leading to inventory reduction), and design rationalization of steel body bits as well as matrix body bits.